This invention relates to a high-sensitivity and compact SQUID magnetometer.
There have been numerous disclosures of magnetometers which employ a superconducting quantum interference device (hereinafter abbreviated as SQUID). For example, a recent article in the Shimadzu Review (Vol. 41, June, 1984, pp. 99-113, in Japanese) disclosed a triaxial SQUID magnetometer for use in measuring undersea geomagnetic fields. As shown in FIG. 1, which is a schematic perspective view of that magnetometer, an x-axis superconducting shield 1, a y-axis superconducting shield 2, and a z-axis superconducting shield 3 respectively house an unillustrated x-axis SQUID, a y-axis SQUID, and a z-axis SQUID. The three SQUID's are respectively driven by an x-axis SQUID drive circuit 4, a y-axis SQUID drive circuit 5, and a z-axis SQUID drive circuit 6. An x-axis pickup coil 7, a y-axis pickup coil 8, and a z-axis pickup coil 9 are mounted on a bobbin 13. The three pickup coils 7-9 lie in three mutually orthogonal planes, the x-axis pickup coil 7 lying in a plane which is normal to the x-axis in FIG. 1, the y-axis pickup coil 8 lying in a plane which is normal to the y-axis, and the z-axis pickup coil 9 lying in a plane which is normal to the z-axis. The unillustrated x-axis SQUID which is housed within the x-axis superconducting shield 1 is electrically connected to the x-axis pickup coil 7 by superconducting twisted wires 10. Similarly, the y-axis SQUID and the z-axis SQUID are respectively connected to the y-axis pickup coil 8 and the z-axis pickup coil 9 by superconducting twisted wires 11 and 12, respectively.
FIG. 2 schematically illustrates the elements which are housed within the x-axis superconducting shield 1. A capacitor 14 is connected between the x-axis SQUID drive circuit 4 and ground, while an inductance 15 is connected in parallel with the capacitor 14. An rf current is passed through the inductance 15 by the drive circuit 4. The inner ends of the superconducting twisted wires 10 are connected to an x-axis input coil 17. The x-axis pickup coil 7, the superconducting twisted wire 10, and the x-axis input coil 17 form a single superconducting closed loop. An x-axis rf SQUID 16 is disposed inside the superconducting shield 1 and is magnetically coupled with the inductance 15 and the input coil 17. The SQUID 16 has a single Josephson tunnel junction 16a. The y-axis superconducting shield 2 and the z-axis superconducting shield 3 house similar components.
When the conventional magnetometer of FIGS. 1 and 2 is operated, the superconducting shields and the components housed therein are cooled to a cryogenic state in which they become superconducting. With reference to FIG. 2, quantization of flux takes place in the closed superconducting loop formed by the x-axis pickup coil 7, the twisted wires 10, and the input coil 17, and a flux which threads this closed loop is always maintained constant. Therefore, if the flux which threads the x-axis pickup coil 7 should vary with respect to its value when the closed loop became superconducting, a shielding current which acts to maintain the flux constant within the closed loop will flow through the closed loop, and a change in flux will be transmitted from the x-axis input coil 17 to the x-axis rf SQUID 16. The x-axis rf SQUID 16 is magnetically coupled with the inductance 15, so any change in the flux through the x-axis pickup coil 7 will produce a change in the output voltage Vout(x) of the x-axis SQUID drive circuit 4. Due to the Meissner effect, the x-axis superconducting shield 1 is completely diamagnetic when in a superconducting state, and external noise is prevented from reaching the x-axis rf SQUID 16.
The circuits for the y-axis SQUID and the z-axis SQUID operate in the same manner. By means of this apparatus, the components along the x, y, and z axes of a magnetic field can be detected.
In a conventional SQUID magnetometer of this structure, the external magnetic field is distorted in the vicinity of the superconducting shields 1-3. This distortion produces inaccuracies in the measured field strength. It is posible to reduce the influence of the distortion produced by the superconducting shields 1-3 by increasing the distance between the superconducting shields and the pickup coils 7-9, but this causes an undesirable increase in the size of the magnetometer.